CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net income	$ 124,080	$ 138,237	$ 151,099
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	38,547	31,189	28,723
Amortization of marketable securities	884	783	(486)
Stock-based compensation	40,623	60,145	23,993
Deferred income taxes	4,711	(5,647)	4,860
Changes in operating assets and liabilities:
Accounts receivable	(120,560)	73,033	(65,156)
Prepaid insurance	(1,121)	254	(2,007)
Other current assets	(13,851)	4,032	(15,772)
Other assets and ROU assets	4,356	15,725	9,722
Accounts payable	209	1,638	(6,057)
Payroll taxes and other payroll deductions payable	89,932	100,712	16,082
Accrued worksite employee payroll expense	74,817	(67,023)	71,880
Accrued health insurance costs	17,316	11,505	(13,973)
Accrued workers’ compensation costs	(197)	(3,052)	13,247
Accrued corporate payroll, commissions and other accrued liabilities	14,716	(20,763)	(6,386)
Income taxes payable/receivable	(14,307)	5,585	(4,616)
Total adjustments	136,075	208,116	54,054
Net cash provided by operating activities	260,155	346,353	205,153
Marketable securities:
Purchases	(58,202)	(50,624)	(110,131)
Proceeds from maturities	60,045	49,635	108,380
Proceeds from dispositions	0	484	28,313
Property and equipment:
Purchases	(32,907)	(98,159)	(56,307)
Proceeds from dispositions	51	43	21
Net cash used in investing activities	(31,013)	(98,621)	(29,724)
Cash flows from financing activities
Purchase of treasury stock	69,725	99,415	203,043
Dividends paid	(144,179)	(61,869)	(48,622)
Borrowings under long-term debt agreement	0	100,000	125,000
Other	5,831	7,701	8,312
Net cash used in financing activities	(208,073)	(53,583)	(118,353)
Net increase in cash and cash equivalents	21,069	194,149	57,076
Cash, cash equivalents and restricted cash beginning of year	786,699	592,550	535,474
Cash, cash equivalents and restricted cash at end of year	807,768	786,699	592,550
Supplemental disclosures of cash flow information
Income taxes, net	53,835	51,097	38,299
Cash paid for interest	7,268	8,977	7,421
ROU assets obtained in exchange for lease obligations	19,572	22,770	24,474
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents [Abstract]
Cash and cash equivalents	575,812	554,846	367,342
Restricted Cash and Cash Equivalents, Current	46,929	45,522	49,295
Deposits workers' compensation	$ 185,027	$ 186,331	$ 175,913